Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water or fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis. An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z-axis and that varies linearly in amplitude with position along one of the x, y, or z-axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and, in turn, on the resonant frequency of the nuclear spins along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MRI signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spins in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and know reconstruction algorithms.
The design of a gradient coil typically involves many compromises. For example, it is desirable to have a gradient coil that produces a highly linear magnetic field while using the smallest amount of current from a power supply. Additionally, gradient coils may necessitate cooling in order to avoid patient discomfort due to heat generation in the gradient coils. In some examples, gradient coils may be comprised of hollow wire in order to provide a conduit within the gradient coils for carrying coolant. To increase the strength of the magnetic field, it may be desirable to include a higher number of turns within a gradient coil. However, simply utilizing smaller diameter wire in the gradient coil increases resistance of the gradient coil and places additional load on the gradient coil current driver. Further, the inclusion of hollow gradient coils increases the diameter of the wire comprising the gradient coil and constrains the number of turns the coil is capable of including.